Chemistry Reference
In-Depth Information
of Novozym 435, will lead to a ring-opened product with an
-alcohol chain
end. This product is virtually unreactive and propagation does not occur. To en-
able polymerization of
(
S
)
(
S
)
-6-MeCL, racemization of the
(
S
)
-alcohol that is formed
upon ring-opening is required, furnishing a reactive
(
R
)
-chain end. These can
subsequently react with another molecule of
-
6-MeCL is incorporated (which will occur because the selectivity for lactones is
moderate with an
E
of 12), a reactive
(
S
)
-6-MeCL. If the less reactive
(
R
)
-chain end is obtained and propagation
occurs instantly. In this way, both enantiomers of the monomer are consumed. It
appeared, however, that polymerization with the Novozym 435/
1
catalytic system
was not feasible: only oligomers were obtained in one-step reactions because the
catalysts were poorly compatible under the reaction conditions employed.
was employed in combination with the addition of DMP to suppress dehydrogena-
(
R
)
-6-MeCL, with a promising
ee
of 86% and
M
p
of
8.2 kDa, was obtained after workup starting from optically pure
(
R
)
-6-MeCL. The
low rate of reaction compared to DKR (typically complete after 48 h with the Shvo
catalyst) is attributed to the low concentration of the terminal alcohol as well as to
the iterative nature of the system. Racemic 6-MeCL showed comparable rates of
reaction for both enantiomers, which polymerized within 220 h with complete con-
version of both enantiomers, yielding polymers of high
ee
(92%) and
M
p
(9.4 kDa).
Successful polymerizations with more than 100 consecutive and iterative enzymatic
additions and Ru-catalyzed racemizations on one polymer chain were realized.
(
S
)
3.4.4
Chiral Block Copolymers by Chemoenzymatic Catalysis
Chiral polymers can be prepared using a one-pot system, i.e., all reactants and cata-
lysts are present at the start of the reaction and both catalysts work simultaneously.
However, one can also envisage the synthesis of chiral polymers using catalysts
in sequence, either in one pot or even completely independent of each other. This
section will deal with the synthesis of chiral block copolymers using different
catalysts in sequence. An interesting example of the synthesis of chiral polymers
using catalysts in sequence is the synthesis of chiral block copolymers in a sequen-
tial approach. Both ATRP and nitroxide-mediated LFRP were evaluated for this
purpose.
Peeters et al. combined the enzymatic ring-opening polymerization of 4-MeCL
of a chiral block copolymer. In the case of combining ATRP and enantioselective
ROP (eROP) of 4-MeCL, it was found that the addition of Ni
PPh
3
)
4
inhib-
ited Novozym 435 and at the same time catalyzed the ATRP reaction. While
Novozym 435 did not interfere with the ATRP of MMA, Ni
(
PPh
3
)
4
clearly inhib-
ited Novozym 435 during the eROP of 4-MeCL. After precipitation, the chiral block
copolymers (
M
n
=
(
11 kDa and 17 kDa) were isolated as solid compounds showing
two glass transition temperatures
60 and 100
◦
C, indicative of phase sepa-
(
T
g
)
at
−
ration between the two blocks.
